The present invention relates to a method of controlling a continuously variable transmission.
A method of controlling a continuously variable transmission is known from Japanese patent application laid-open publication No. 57-161346 which has a U.S. counterpart, i.e., U.S. Pat. No. 4,543,077, issued to Yamamuro et al., on Sept. 24, 1985. According to this known method, there is stored a predetermined pattern of correspondence between throttle opening degree and vehicle speed. The predetermined pattern is retrieved in accordance with detected values in throttle opening degree and in vehicle speed in order to give a desired revolution speed value driving pulley. The transmission for the reduction ratio is controlled by a shift motor in so as to achieve the desired driving pulley revolution speed value. The predetermined pattern stored is illustrated in FIG. 10. According to this pattern, a predetermined value of driving pulley revolution speed (i.e., engine speed) corresponds to one degree in throttle opening, and the same value in driving pulley revolution speed is maintained for the same throttle opening degree irrespective of how high the vehicle speed is.
With this pattern, the transmission cannot meet a rapid acceleration demand which occurs during a shift, initiated after a rapid increase in engine throttle opening, from a stable running state at substantially the same vehicle speed with a relatively small throttle opening. During a transient period until a new target reduction ratio is attained which corresponds to a new target engine speed that is set after the rapid increase in engine throttle opening, the transmission continuously shifts, thus requiring a long time before the new target reduction ratio is attained. Thus, during this transient period, the engine begins to race because the engine speed increases when the reduction ratio increases, thus resulting in a delay in an increase in driving force resulting in a sluggish response to the rapid acceleration demand.
Describing in more detail referring to FIGS. 8 to 10, let us now assume that a rapid increase in engine throttle opening occurs under a condition expressed by a point A in FIG. 10 to cause a change in target engine speed to a new target engine speed as indicated by a point B'. After this change in target engine speed, the actual engine speed rises as shown by the dashed line in FIG. 8 to reach the target engine speed at the point B'. Since, during this transient period, the engine is about to race, the engine cannot increase its driving force immediately. As shown by the dashed line in FIG. 9, an increase in the driving force is gradual initially until it increases rapidly after the completion of shifting operation in the transmission at the point B'. Thus, during a transient period between the instants t.sub.0 and t.sub.2, an acceleration feel is not provided, thus deteriorating the driving feel.
The present invention aims at solving the above mentioned problem by providing a quick response to an acceleration demand so as to enhance the driving feel.